1. Field of the Invention
The present invention is related to a speed control circuit for a brushless dc motor. More particularly, the present invention is related to a speed control circuit of pulse-width modulation (PWM).
2. Description of the Related Art
Referring to FIG. 1, a conventional fan having a speed-adjustable drive circuit discloses a current-controlling circuit 1, a PWM drive IC member 2, and a PWM input terminal 3 connected to the current-controlling circuit 1. The current-controlling circuit 1 includes a transistor (Q4) and a resistor (R5). The transistor (Q4) has a collector electrically connected to two emitters of two transistors (Q1, Q2), and an emitter connected to ground. The two transistors (Q1, Q2) have two collectors serially connected to two coils (V1, V2) respectively. Inputting a PWM signal, it can turn on or off a base of the transistor (Q4), thereby controlling to actuate or cut off the coils (V1, V2) alternatively.
The PWM drive IC member 2 is adapted to electrically connect to two bases of the two transistors (Q1, Q2), thereby controlling to turn on or off the two transistors (Q1, Q2) alternatively. Then, each of the two transistors (Q1, Q2) is able to conduct between the collector and the emitter synchronously. In operation, a current, which is supplied from a power source, is controlled to pass through the coils (V1, V2) by means of saturation or unsaturation between the collectors and the emitters of the two transistors (Q1, Q2). Thus, the coils (V1, V2) are alternatively controlled to allow passage of a current of a power source according to frequency of the PWM signal.
However, during controlling speed of the motor, the transistor Q4 must allow and bear a passage of a huge current that may be passed through the coils (V1, V2) or cut off. And thus there is a need for using a big transistor. Consequently, it may increase manufacture cost of the fan. Moreover, it results in a destruction of overheating the transistor Q4 after a long-term successive operation by high on/off frequency of the huge current from the power source controlled under a PWM signal. Consequently, it may shorten useful life of the fan.
Furthermore, the speed-adjustable drive circuit must employ an expensive semiconductor component of the PWM drive IC member 2 that may increase manufacture cost.
The present invention intends to provide a speed control circuit for a brushless dc motor having a PWM control circuit connected between plural output terminates of a drive member and plural transistors of a motor drive circuit. The PWM control circuit is adapted to employ PWM signals for simply controlling driving signals generated from the drive member that is in control of turning on or off a transistor of the motor drive circuit in such a way to mitigate and overcome the above problem.
The primary objective of this invention is to provide a speed control circuit for a brushless dc motor having a PWM control circuit connected between plural output terminates of a drive member and plural transistors of a motor drive circuit. The PWM control circuit is adapted to control driving signals generated from the drive member by employing PWM signals so that a transistor of the motor drive circuit may be turned on or off. Thereby, it may prolong useful life of the motor.
The secondary objective of this invention is to provide a speed control circuit for a brushless dc motor having a PWM control circuit connected between plural output terminals of a drive member and plural transistors of a motor drive circuit. Use of the PWM control circuit to only control the driving signal of the drive member results in low manufacture cost, and the PWM control circuit has compatibility with the original motor drive circuit. Thereby, it may reduce manufacture cost of the motor.
The speed control circuit for a brushless dc motor in accordance with the present invention comprises a motor drive circuit, a drive member and a PWM control circuit. The motor drive circuit includes at least one coil and at least one transistor serially connected to the coil. The drive member is provided with a plurality of output terminals adapted to output a driving signal to the transistor of the motor drive circuit. The PWM control circuit is in connection between the transistor of the motor drive circuit and the output terminal of the drive member, thereby turning the transistor on or off.
Other objectives, advantages and novel features of the invention will become more apparent from the following detailed description and the accompanying drawings.